Polyunsaturated fatty acids (hereinafter referred to as PUFAs) have a variety of useful physiological functions. PUFAs herein refer to fatty acids containing 20 or more carbon atoms and having two or more double bonds. In recent years, PUFAs, in particular, dihomo-gamma-linolenic acid (hereinafter referred to as DGLA) and arachidonic acid (hereinafter referred to as ARA) have been found useful in varied ways (Non-Patent Literature 1). For example, it has been found that DGLA has an inhibitory effect on atopic dermatitis and an antiallergic effect and ARA has a brain function improvement effect and a nutrient effect for infants. In addition, there is concern about lack of DGLA and ARA in patients afflicted with adult diseases including potential patients, infants, elderly people, and pets (in particular, animals of the feline family).
Under such circumstances, various investigations on supply of PUFAs, in particular DGLA or ARA, have been carried out. Culturing microorganisms capable of producing such fatty acids has been studied as practical processes. Various investigations on such processes have also been carried out to find a process suitable for industrial production (Non-Patent Literature 1). For PUFAs (in particular, DGLA), a large-scale method of culture was found that yielded up to 167 g of DGLA for 1 kg of dry microbial cell (Patent Literature 1 and Non-Patent Literature 2). In these methods, however, division of the culture medium and addition of glucose must be frequently repeated for at least several days from the beginning of the culture in order to avoid low productivity due to consumption of a large amount of glucose during the beginning of the culture and growth inhibition that results from high glucose concentration.
Furthermore, methods of culture for microorganisms have been studied to improve the productivity of particular substances. Patent Literature 2 discloses a method for culturing Kluyveromyces lactis in a culture medium containing whey to produce cerebroside by fermentation, which includes addition of various sodium sources to the culture medium, and control of the pH of the culture medium.
In addition, with culture of the genus Mortierella (filamentous fungus) Non-Patent Literature 3 discloses a method for controlling the pH using NH4OH, which is also used as a nitrogen source, whereas Patent Literature 3 discloses control of the pH within the range of 7 to 7.5 in the latter half of the culture period. Non-Patent Literature 4 discloses addition of 0.05% CaCl2.2H2O, 0.05% MgCl2.6H2O, and 0.1% Na2SO4 together with KH2PO4 to a culture medium leads to fungus in an optimal form and an increase in the productivity of PUFA.
[Patent Literature 1] Japanese Patent No. 3354581
[Patent Literature 2] JP-A-2006-55070
[Patent Literature 3] U.S. Pat. No. 5,658,767
[Non-Patent Literature 1] Hiroshi Kawashima, Foods Food Ingredients J Jpn, 210, 106-114 (2005)
[Non-Patent Literature 2]H Kawashima et al., J Am Oil Chem Soc, 77, 1135-1138 (2000)
[Non-Patent Literature 3] Byung-Hae Hwang et al., Biotechnol Lett, 27, 731-5 (2005)
[Non-Patent Literature 4] Shigeaki Fujikawa et al., Bioscience and Industry, 57, 818-821 (1999)